Many polypeptides that are naturally immunogenic lose this property when expressed recombinantly. In some cases the native polypeptide has structural features which do not form during expression in a heterologous host e.g. post-translational modifications may be incorrect, intermolecular interactions which influence conformation may be lost, etc. A further cause of lost immunogenicity is where a polypeptide (e.g. a surface-exposed polypeptide) is naturally oligomeric, and where this quaternary structure is required for immunogenicity (e.g. where the polypeptide has epitopes that are displayed only when a specific quaternary oligomeric structure is present). Loss of oligomeric structure can mean that the monomeric protein is less immunogenic than its native oligomeric counterpart.
In other cases the native polypeptide may be a transmembrane polypeptide that is not amenable to expression in a recombinant host. These problems are often seen when eukaryotic polypeptides (including those of eukaryotic viruses) are to be expressed in prokaryotes. One way of improving expression of viral transmembrane polypeptides is to remove their transmembrane domains and express only the antigenic extracellular domains {1}. However, this “soluble receptor” technology again suffers from loss of quaternary structure. If a native receptor exists in an oligomeric form on the surface of a virus, and the oligomerisation arises from sequences in the transmembrane region, the soluble receptor will lose its ability to oligomerise, and this loss can have functional consequences e.g. loss of signalling or of avidity. Loss of binding avidity, even though binding affinity may be retained, is a particular problem for antigens e.g. those used in vaccines.
Techniques for oligomerising proteins have been disclosed in references 2 and 3.
It is an object of the invention to provide ways of improving the expression of polypeptides, and particularly of antigenic polypeptides e.g. to retain their oligomeric structure.